Freeze-fracture electron microscopy has revealed organized sites of aggregated intramembranous particles in granular cell luminal membranes from toad urinary bladder after vasopressin treatment. This response appears to be a primary hormonal effect which is reversible and quantitatively and specifically related to induced alterations in membrane water permeability. The proposed research is to learn about the aggregation response in terms of formation and disassembly processes, to determine whether sites of aggregation are actual sites for water passage, and to determine if a similar process may occur in the mammalian kidney.